Conversion of sunlight into electricity usually takes one of two forms: the “quantum” approach using the large energy of solar photons in photovoltaic (PV) cells, or the “thermal” approach using solar radiation as the heat source in a classical heat engine. Quantum processes boast high theoretical efficiencies as the effective photon “temperature” is Tsolar˜5800° C., yet suffer in practice from a limited spectral energy collection window and thermalization losses. Thermal processes take advantage of energy throughout the entire spectrum, but efficiency is curbed by practical operating temperatures. Combinations of the two are predicted to have efficiencies >60%, yet fail in practice because PV cells rapidly lose efficiency at elevated temperatures, while heat engines rapidly lose efficiency at low temperatures. As a result, these two approaches remain disjointed.
Hot-carrier solar energy converters provide a helpful example of the difficulties typically encountered in combining quantum and thermal conversion approaches. In hot-carrier solar energy converters, efficiency is improved by having photo-generated electrons be emitted from a cathode before thermalization of the generated electrons with respect to the cathode can occur. If this can be accomplished, efficiency can be significantly increased, because a significant source of loss (i.e., thermalization in the cathode) is thereby mitigated. However, typical thermalization time scales in condensed matter are on the order of picoseconds, so it is extremely difficult to provide high-efficiency emission of non-thermalized (i.e., hot) electrons.
Accordingly, it would be an advance in the art to provide combined thermal and quantum conversion that can more readily be realized in practice.